1. Field of the Invention
Exemplary embodiments of the present invention relate to a network architecture of a high-speed optical network system, and in particular, to a hybrid wavelength division multiplexing (WDM)/time division multiplexing (TDM) passive optical network (PON) system using an orthogonal frequency division multiplexing (OFDM)-based optical modem as a main method of transfer.
2. Discussion of the Background
A passive optical network (PON) is mainly built from a passive device having low power consumption, and uses an optical fiber as a main transmission medium. The PON may maximize a physical transmission bandwidth, and may ensure economical maintenance and repair, from a perspective of a service provider, due to the use of a passive device as a subscriber distributing means. Also, the PON has an advantage of being able to receive various existing multilevel modulated signals for improving a transmission bandwidth. Here, the PON corresponds to a most powerful candidate for a future optical subscriber network.
To date, research has been mainly directed to develop an optical transmitter and an optical receiver configured to enable transmission and reception across an optical link for upstream and downstream transmissions in an effort to commercialize usage of a PON. An optical transmitter and an optical receiver used in a conventional PON may enable upstream and downstream transmissions using a relatively expensive optical device, for example, a laser diode, an external modulator, and the like. For these reasons, upgrading of a system to meet additional bandwidth requirements is usually achieved in an aspect of hardware, which may impose an economical burden on a subscriber and may be an obstruction to system expansion and network re-construction and dynamic resource allocation for traffic control for efficient use of transmission resources.
In particular, a simple increase in a physical bandwidth of an optical transmitter and an optical receiver with increasing bandwidth required per subscriber may result in chromatic dispersion or polarization mode dispersion, and as a result, additional physical layer compensation technologies need to be provided, as well as development of a light source or an optical modulator allowing fast modulation. These additional technologies may result in a high capital expenditure.
To resolve the preceding issues, studies on an orthogonal frequency division multiplexing (OFDM)-based PON are being actively conducted in Europe and the U.S.A. to implement long-distance high-capacity transmission. Studies associated with an OFDM-based PON are generally centered on high-capacity transmission, for example, a maximum convergence bandwidth of 40 gigabits per second (Gbps) or more and long-distance, for example, 60 kilometers (km) and point-to-multipoint architecture, for example, a maximum of 64 points.
In general, a PON architecture has been implemented and developed through the use of two technologies including a time division multiplexing (TDM)-PON technology allowing upstream and downstream transmissions by allocating a time frame to each subscriber, and a wavelength division multiplexing (WDM)-PON technology allowing upstream and downstream transmissions by allocating a predefined specific wavelength to each subscriber.
Currently, a TDM-PON technology has a maximum upstream and downstream convergence bandwidth of 10 Gbps, and in a case of 64 points, has an average bandwidth per subscriber of about 150 megabits per second (Mbps). The TDM-PON technology has an advantage of adaptively responding to a change in bandwidth required per subscriber since the TDM-PON technology provides a dynamic bandwidth allocation function, but has a disadvantage of having to change optical transmitter and optical receiver hardware when a maximum convergence bandwidth is increased.
Conversely, a WDM-PON technology currently has an average bandwidth of about 1 Gbps per subscriber and, in principle, may have a maximum convergence bandwidth of up to 40 Gbps. However, a WDM-PON technology has disadvantages of lower network flexibility and a higher average installation cost per subscriber than a TDM-PON technology in that a bandwidth provided to each subscriber is determined based on a wavelength and a physical bandwidth limit of a light source allowing fast modulation. Also, an issue associated with additional physical layer compensation technologies for providing a high-speed bandwidth per wavelength will be happened.